In an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, an electrostatic image formed is first developed with a toner for electrostatic image development (hereinafter may be referred to as a toner merely). After the toner image formed is then transferred to a transfer medium such as paper or OHP film, the unfixed image is fixed thereto by any of various methods such as heating, pressing and use of solvent vapor.
A toner for electrostatic image development is generally composed of colored polymer particles (colored resin particles) comprising a binder resin and a colorant. Processes for producing the toner for electrostatic image development are roughly divided into a pulverizing process and a polymerization process. In the pulverizing process, a colorant, a charge control agent, a parting agent and the like are melted and mixed in a thermoplastic resin to uniformly disperse them therein, thereby preparing a composition, and the composition is then pulverized and classified, thereby producing a toner. In the polymerization process, a monomer composition obtained by uniformly dissolving or dispersing a polymerizable monomer, a colorant, a charge control agent, a parting agent and the like in one another is poured into water or an aqueous dispersion medium composed mainly of water, which contains a dispersion stabilizer, and the mixture is stirred until the droplet diameter of droplets becomes fixed. A polymerization initiator is added to the mixture, and the monomer composition is dispersed by means of a mixer having high shearing force to form the monomer composition into fine droplets. The droplets are then subjected to polymerization, filtration, washing, dehydration and drying, thereby producing a toner. According to the polymerization process, a toner having a desired particle diameter and a sharp particle diameter distribution can be obtained without conducting pulverization and classification.
In copying machines, printers and the like of an electrophotographic system, it has recently been attempted to reduce demand power. A step in which energy is particularly demanded in the electrophotographic system is the so-called fixing step conducted after transferring a toner from a photosensitive member to a transfer medium. A heating roll heated to at least 150° C. is generally used for fixing, and electric power is used as an energy source therefor. It is effective from the viewpoint of energy saving to lower the temperature of the heating roll.
Besides, the speeding-up of copying and printing has been strongly required with the advancement of the combination of image forming apparatus and the formation of personal computer network. In such high-speed copying machines and high-speed printers, it is necessary to conduct fixing in a shorter time.
In order to meet such requirements from the image forming apparatus in the design of a toner, it is only necessary to lower a glass transition temperature of a binder resin. When the glass transition temperature of the binder resin is lowered, however, the resulting toner becomes poor in the so-called shelf stability because particles themselves of the toner undergo blocking during storage or in a toner box to aggregate.
On the other hand, in the case of color toners used in the electrophotographic system, development is generally conducted with color toners of 3 or 4 different colors to transfer the resulting toner image to a transfer medium at a time or by 3 or 4 installments, and the toner image is then fixed. Therefore, the thickness of the toner layer to be fixed becomes thicker compared with a black-and-white image. In addition, the respective color toners overlapped are required to be uniformly melted, and so the melt viscosity of each toner at about the fixing temperature thereof must be designed low compared with the conventional toners. Means for lowering the melt viscosity of the toner include, for example, methods in which the molecular weight of a resin used is made lower compared with the resins for the conventional toners, and in which the glass transition temperature thereof is lowered. In any of these methods, however, the toner becomes poor in shelf stability because the toner tends to undergo blocking.
As described above, there is an adverse correlation between the shelf stability of a toner and the means for coping with the lowering of the fixing temperature of the toner, the speeding-up of printing and the formation of color images.
On the other hand, Japanese Patent Application Laid-Open No. 59-62871 has proposed a positively charged polymerized toner making use of a nigrosine dye as a charge control agent. However, the nigrosine dye is not suitable for use in color toners because its color is black though it exhibits excellent charge control property in a small amount.
In order to solve this problem, Japanese Patent Application Laid-Open No. 59-123852 has proposed a process of subjecting a polymerizable monomer and a cationic polymer to suspension polymerization in an anionic dispersing agent, Japanese Patent Application Laid-Open No. 63-60458 a toner obtained by a pulverizing process making use of a charge control resin composed of a quaternary ammonium salt-containing copolymer, and Japanese Patent Application Laid-Open No. 03-175456 and WO 99/47982 a production process of a toner according to a polymerization process in which a colorant and a polymerizable monomer are polymerized in the presence of a quaternary ammonium salt-containing copolymer.
The charge control resins (cationic polymers) specifically described in these publications are high in styrene content. Investigations by the present inventors have revealed that a toner obtained by using a resin having a styrene content of at least 80% by weight as a charge control resin causes fixing failure and deterioration of printing in high-speed continuous printing.